Many modern cellular communication systems such as HSPA (High Speed Packet Access) and LTE (Long Term Evolution) use automatic link adaptation to achieve efficient communication under varying transmission conditions. The effective bitrate is varied quickly, along with related transmission parameters such as code rate and modulation scheme, depending on predicted radio conditions. When the radio conditions get worse, e.g. at increased interference, the bitrate is decreased to reduce the probability of decoding error. Correspondingly, when the conditions improves, e.g. at decreased interference, the bitrate is increased to increase the transmission efficiency without causing a too high error probability. The radio conditions are often predicted based on past measurements of the radio channel.
Another key technology in these systems is HARQ (Hybrid Automatic Repeat reQuest). With HARQ, failure to decode a received transport block results in a retransmission, possibly with a different redundancy version.
In the simplest version of HARQ, Type I HARQ, both error detecting and FEC (Forward Error Correcting) information is added to each message prior to transmission. When a coded data block is received, the receiver first decodes the error-correction code. If the channel quality is good enough, all transmission errors should be correctable, and the receiver can obtain the correct data block. If the channel quality is bad, and not all transmission errors can be corrected, the receiver will detect this situation using the error-detection code, then the received coded data block is rejected and a retransmission is requested by the receiver, similar to ARQ (Automatic Repeat Request).
In a more sophisticated form of HARQ, Type II HARQ, the message originator may, when a first transmission is received error free, exclude the FEC parity bits from consecutive transmissions. Further, information from two consecutive transmissions comprising errors can be combined by a receiver, thereby enabling deriving of an error free transmission result without having received an error-free transmission.
When a receiver fails in its attempt to decode a transport block, it typically stores the received signal, or a processed version thereof, and combines it with a later received signal being a retransmission of that block. This is known as soft combining, and greatly increases the probability of a correct decoding. Variants of soft combining are Chase combining and incremental redundancy.
In many HARQ protocols, the receiver sends a HARQ feedback after each decoding attempt, in the form of a positive or negative acknowledgement (ACK/NACK), to indicate whether the particular transport block was correctly decoded or not. In case a NACK is sent, the transmitter typically retransmits the transport block. In the case of an ACK, the transmitter can instead use its resources to transmit new data, to the same or a different user.
An alternative HARQ protocol arrangement is to let the receiver control the transmissions, as is done on the LTE uplink. The receiver sends a grant for each requested transmission, indicating among other things the transport format, e.g. modulation and code rate, and whether a retransmission or an original transmission is requested. A grant for a retransmission may in some cases consist of a single bit, similar to a HARQ ACK/NACK, but may in other cases be a complete grant of the same size as a grant for an original transmission. With this view, transmission grants can be seen as a kind of HARQ feedback; this is the view we take in this document.
When applying the different variants of HARQ described above, the receiver must perform a complete decoding attempt before it can decide upon its next action, e.g. sending an ACK or a NACK. Modern error-correcting codes, such as Turbo codes, are very complex to decode, resulting in long delays from transmission until a feedback message can be sent back to the transmitter. This results in long round-trip delays of the HARQ retransmissions. For LTE, the minimum round-trip delay is 8 ms (LTE Frequency Division Duplexing).
Because of the nature of radio channels, and the behavior of interference from other transmitters, it is difficult to make an accurate prediction of the radio conditions for a particular transmission. This makes it necessary to apply a significant margin against sudden variations, to keep the probability of decoding error acceptably low. Such a margin reduces the average throughput.
A higher average throughput can theoretically be achieved by using a higher original transmission bitrate and accepting a higher error probability. The problem with this approach is that it results in much longer packet delays, since each retransmission adds one round trip time, 8 ms in LTE FDD, to the total transmission time of the packet.
HARQ with soft combining can be viewed as a kind of implicit link adaptation mechanism. This is the case if the bitrate is chosen so high that one or more retransmissions are often needed. The effective bitrate of the entire transmission of a transport block then depends on the number of transmissions, including original and retransmissions, as well as transport format parameters such as modulation and coderate. Contrary to link adaptation based on past measurements, the effective bitrate of such a HARQ transmission is determined by the radio conditions during the actual transmission of the transport block.
The problems with the current HARQ solutions are many-fold. The HARQ-roundtrip time limits the performance and also gives strict scheduling timing requirements for retransmissions. Further, as we move towards more and more diverse implementations with carrier aggregation, HetNet deployments, self backhauling and machine to machine communication etc, a need for HARQ-less operation can arise e.g. because of complexity of implementation of HARQ-operation.